External Fixators for the Treatment of Proximal Tibia Fractures: Ringed and Standard

Introduction

Tibial plateau fractures come in many forms from the senile nursing home patient falling on the way to the bathroom to the young motorcyclist driving into a tractor-trailer. Similarly, external fixators range from simple uniplanar large pin frames to complex ring fixators that take hours to construct and “fine tune.” This chapter is a guide to fitting the frame to the problem.

The treatment of bicondylar tibial plateau fractures can be a daunting undertaking. The orthopedic trauma surgeon must keep in mind that the amount of energy transferred to the bone also takes its toll on the soft tissues. Not only can the injury to the cartilage and the subchondral plate lead to arthrosis but the injury to the soft-tissue envelope can also cause many complications involved with open reduction and plate fixation of these fractures. Oftentimes, treatment requires dual plating, which in itself involves extensive mobilization of the already severely injured soft tissues from the comminuted bone with impaired blood supply, walking the fine line of being able to heal as well as not becoming infected.

Ringed external fixators have previously been utilized in the treatment of these high-energy injuries but have the potential for the added complication of a septic knee joint from intraarticular pins. Recent modifications to pin placement based on anatomic studies. have led to a resurgence in the use of ringed fixators for the treatment of these devastating injuries. This chapter will guide you through pin placement and configuration for a ringed fixator as well as simple large pin external fixators.

Biomechanics

The construction of the ring external fixator allows the surgeon to adjust the stiffness of the construct as well as to customize the fixation based on the injury pattern. With adjustments to the transfixion wire spread, the crossing angle, ring diameter, half-pin diameter, number of rings, and the ring symmetry the surgeon can adjust the stiffness of the construct. All of these variables can be used to increase or decrease the stiffness and, therefore, the overall stability of the construct.

A crossing angle of the tensioned wires in the proximal tibia increased from 30 to 90 degrees will increase the axial, torsional, and bending stiffness by 75%. ^, Transfixion pin angles between 60 and 90 degrees allow appropriate stiffness and resistance to shear forces in the sagittal plane with knee range of motion ^{, ,} but can lead to the placement of pins outside of the soft-tissue safe corridors. An additional half-pin placed in the anterior position in the proximal tibia will augment sagittal stability. Gellar et al. changed the pin configuration so that the pins did not all cross the tibia in the center, allowing for crossing angles of 80 degrees, while placing them more within the sagittal plane. However, this places the pins outside the soft-tissue safe corridors ^, and can lead to impingement on the patellar tendon. Antoci et al. experimented with varying wire-crossing configurations while maintaining 60-degree crossing angles. Wire placement with two pins crossing 1 cm posterior to the center of the plateau and the horizontal wire in the coronal plane passing 1 cm anteriorly and inferiorly from the center of the tibia increased the sagittal plane stiffness without decreasing coronal or torsional stiffness ( Fig. 4.1 ). Three wires are sufficient for weight bearing and stiffness of the construct, but more wires may be used if desired to further increase stability. Wires 1.8 mm in diameter are small enough to capture smaller fragments of comminution as well as stiff enough once tensioned to resist loads ; however, the larger the diameter, the greater the stability imparted. Tensioning olive wires on both sides of the fracture and placing them on the side of bending will also increase the bending stiffness of the external fixator. Tensioned wires cannot be placed in the anterior-posterior direction as this would place the posterior tibial artery, nerve, and muscles at risk of injury. The greatest deforming force is in the sagittal (anteroposterior [AP]) plane when walking, and half-pins apply greater frame stiffness in this plane and are therefore used to add to the stability in the sagittal plane. ^, A second ring added to the proximal segment as well as wires in two levels will also increase the stability of the construct. ^, Hybrid fixators follow similar principles with the addition of half-pins in the diaphysis where the half-pins have greater purchase. Using pins mounted on rings will also increase the overall stiffness. ^,

The stability of traditional knee-spanning constructs with half-pins and bars is less critical as they are, in general, temporary in nature. Half-pin diameter, the number of half-pins, multiplanar half-pins, bar height relative to the bone, pin to fracture distance, and the working length of the fixator are all variables that can be adjusted to manage the stiffness of the montage. ^,

Indications

The general indication for external fixation of a proximal tibia fracture is a complex fracture with soft-tissue injury, which would make open plating unsafe due to increased risk of infection and/or further soft-tissue injury. Fracture patterns can be used as an indication for external fixation; more specifically, Schatzker type V and VI fractures, proximal tibia fractures with metaphyseal and subchondral comminution not amendable to routine plating, soft-tissue compromise (i.e., compartment syndrome), mangled limbs, fractures with soft-tissue defects, and fractures in the multiply injured patient. ^{, , , ,} Schatzker type IV fracture may also be considered for external fixation. These can be deceptively high-energy fractures with compromise of the soft tissue, knee dislocation, or neurovascular injury. ^,

X-ray clues that staged management may reduce complications include the following:

• 1.

  The mechanical axis of the tibia is not in line with the femur—there is significant varus, valgus, or displacement anteriorly or posteriorly.

• 2.

  The spread of the tibial condyles is wider than the spread of the distal femoral condyles.

• 3.

  The fibular head is disassociated from the tibia.

• 4.

  The tibial tuberosity is pulled anteriorly by the patella ligament.

• 5.

  There is air in the proximal tibia or the interosseous space.

• 6.

  A part of the plateau is displaced posteriorly.